A combined vehicle brake is known from DE 102008051350 A1 that describes a hydraulically operable service brake and an electromechanically operable parking brake, also known as a fixing brake, wherein a hydraulic working pressure chamber within a brake housing is bounded by a brake piston, which can be subjected to hydraulic pressure medium on the one hand to perform service braking and on the other hand can be operated along a longitudinal axis of a piston by means of an irreversible transmission to achieve parking braking, and wherein the irreversible transmission converts the rotational motion of an electric motor into a translational motion of the brake piston to perform the parking brake process and holds the brake piston in the activated position by the self-locking of the transmission.
Such a parking brake system also includes an electronic controller, which generally corresponds to the driving dynamics controller (DDC controller) of the motor vehicle and contains the known hydraulic and electric components as well as additionally two full bridges for independent control of two direct current motors of the two parking brakes on the rear wheels of the motor vehicle.
With such a parking brake system, customer and safety requirements regarding price, performance, noise, safety, availability and comfort are to be achieved, which are often in conflict with each other.
The design of such systems must therefore be selected such that under normal ambient conditions, e.g. regarding the voltage of the on-board electrical system and the temperature of the electric motors of the parking brake, the necessary application force, which must reach a minimum value, can be reliably applied. For cost reasons, the application force is not directly measured for this purpose, but is determined indirectly from other parameters representing the application force. Thus e.g. for applying the electric parking brake described above a predetermined maximum or switch-off current of the electric motor is used as a switch-off criterion, whereby it is assumed that the target application force of the limit value is achieved as a target force value.
However, this method cannot guarantee safe stopping of the vehicle in all situations, because e.g. at increased temperature and/or low operating voltage the desired switch-off current is no longer achieved.
To solve the problem it is known e.g. from DE 10 2010 029 391 A1 to use the vehicle hydraulics in support of achieving the necessary application force. For this purpose, prior to the application process on the parking brake, it will be decided whether a hydraulic boost is necessary using existing temperature values on the brake actuators and using acceleration values of the wheels. According to the known method according to DE 10 2010 029 391 A1, the hydraulic boosting brake pressure is generated such that in the event of operating the parking brake pressure medium is initially taken from the front wheel brakes and transferred to build up a boosting brake pressure in the parking brakes on the rear wheels, and only then is application of the parking brake performed.
With the known method of hydraulically boosting the parking brake function, prior to applying the parking brake it must be decided whether a hydraulic boost is necessary. This leads to the disadvantage that too often a hydraulic boost is performed although it would not have been necessary. Such behavior represents a significant loss of comfort for the driver.
The object of the invention is to specify a method for the hydraulic boosting of an electric parking brake of a vehicle of the above-mentioned type, with which the disadvantages can be avoided.
This object is achieved by a method with the features described herein.